The last one was Bengali Ottenau in the log above. Deleting this does not help.
I removed the carddav account in davdroid and readded it. Error gone.
The only thing that I did that may can corrupt the database is the SDMaid app.
This app has the option to “optimize” the database. I tried to reproduce the error running sdmaid again but no problem wiht davdroid yet.

This bug just hit me and was gone after deleting the affected contact from the server. I believe i triggered the issue by adding the contact first and moving it into a contacts group using the ownCloud webinterface later after syncing.

android.database.sqlite.SQLiteConstraintException: contacts.pinned may not be NULL (code 19)
at android.database.DatabaseUtils.readExceptionFromParcel(DatabaseUtils.java:173)
at android.database.DatabaseUtils.readExceptionWithOperationApplicationExceptionFromParcel(DatabaseUtils.java:160)
at android.content.ContentProviderProxy.applyBatch(ContentProviderNative.java:512)
at android.content.ContentProviderClient.applyBatch(ContentProviderClient.java:377)
at at.bitfire.davdroid.resource.LocalCollection.commit(LocalCollection.java:327)
at at.bitfire.davdroid.resource.LocalAddressBook.commit(LocalAddressBook.java:156)
at at.bitfire.davdroid.syncadapter.SyncManager.pullChanged(SyncManager.java:210)
at at.bitfire.davdroid.syncadapter.SyncManager.synchronize(SyncManager.java:91)
at at.bitfire.davdroid.syncadapter.DavSyncAdapter.onPerformSync(DavSyncAdapter.java:146)
at android.content.AbstractThreadedSyncAdapter$SyncThread.run(AbstractThreadedSyncAdapter.java:259)